The large shaft excavation industry has basically been the domain of the mining and tunneling industry. Typical shaft diameters generally range from a diameter of about than 8 feet to diameters greater than 50 feet. Typical depths of the shafts can be 50 feet to several thousand feet deep.
Some conventional ways of excavating large diameter shafts are as follows. In one conventional method, miners enter the shaft and excavate the material using mechanical equipment such as road headers. In another method, blasting and hand spading or combinations of the above are used.
The excavated material is then loaded into a muck bucket by a small loader or by hand. The bucket is hoisted above the miners to the top of the shaft and is then dumped. The excavating, loading, hoisting and dumping methods are then repeated until the desired diameter and depth of the shaft is reached. During the operation, shaft wall support is added if needed and the cycle is started over.
The foundation drill shaft industry excavates shafts from the surface. The cylindrical shafts may be filled with concrete and reinforcing steel to form structural columns. Shafts may generally range from 12 inches in diameter to 10 feet in diameter. Typical depths may generally range from less than 10 feet and up to 100 feet. The shaft types excavated by both industries may overlap. Drillers may use a large foundation style drill rig to drill larger shafts. The large diameter shafts may be done by first boring or digging a pilot shaft that ranges from 8 to 10 feet in diameter. A drill bucket with over-reamer arms is then attached to a kelly bar which is rotated by a drilling rig. It is used to ream the pilot shaft out to the desired diameter. The cuttings of earth may fall into the bucket which is hoisted up and dumped. The cuttings may also fall into the pilot shaft where they are drilled out with either the bucket without the over-reamer arms or an auger.
This foundation drilling method generally works well. However, is subject to the availability of the drilling force or torque, hoisting capacity and depth. When the rate of advancement of drilling rate decreases to a certain point, then conventional mining techniques may become more competitive.
One of the limitations encountered while using the traditional methods used in the foundation drill shaft industry is the drilling force or torque that is required. The reamer arms of an over-reamer are equipped with drag bits. A drag bit basically cuts and rips the material being excavated. This requires great amounts of drilling force to push the bits into the material and cut away and loosen the material. The longer the reamer arm the more drilling force or torque required. Also, to get cutting action on some of the harder materials, more weight on the bits is needed to cut or loosen the harder material being excavated. This will require more drilling force or torque. The available drilling force or torque at a reasonable revolution per minute requires relatively high horse power and affects the rate of advancement or drilling the hole.
Other limitations involve hoisting and depth capacity. Some of the larger foundation drilling rigs have single line hoisting capacity of about 30,000 to 50,000 lbs. The weight of a rotating kelly bar that is strong enough to deliver enough turning force or torque and long enough in order to reach a desired considerable depth will usually exceed allowable line pulls or hoisting capacity. The weight of the muck or earth being lifted from the hole along with the weight of the bucket and reamer arms further adds to the problem of exceeding allowable line pulls. The deeper the shaft, the heavier the kelly bar. Using multiple part lines on a traveling block generally may limit the kelly bar to a single element kelly. Multiple telescoping kelly bars generally may have a single line going to the inner bar. For deeper holes, the weight of the kelly and reaming tools should be kept to a minimum to avoid excess allowable single line pulls. However, kellys and reamers on big shafts tend to be of heavy construction because of the severe service factor they are subjected to during drilling.
Other limitations with the present methods are those with the reamer arms which are generally all drag cutters. The arms are either fixed or retracted inward when being raised or lowered. Fixed cutters work when no shaft wall support means such as casing, shotcrete, rock anchors, etc. of smaller diameter are added as drilling the shaft progresses deeper.
When shaft wall support is added, the arms need to swing or retract inward in some manner to clear the shaft support means. If personnel are to enter the shaft, then certain utilities may be attached to the shaft wall such as compressed air, water, electricity, lighting and ventilation lines. Reamer arms, fixed or retractable, may damage these lines if they are hit during entry or exit. The tool is visually observed in order to signal the drill operator if the arms are properly retracted or extended or if it appears the arms may come in contact with utility lines. At great depths, it may be hard to visually see the tool, if not impossible, because of distance, lighting and fog or dust concentration arising from excavation.
An object of the present invention is to eliminate or minimize the above mentioned limitations and provide other benefits.
It is an objective of this invention to provide a two-piece over-reamer apparatus so that the entire over-reamer need not be raised from the excavation each time cuttings must be dumped from a bucket.
It is another objective of this invention to minimize the weight that is handled by the hoisting device each time the kelly bar and bucket is raised from the excavation.
It is another object of this invention to allow the part of the over-reamer apparatus to remain in the shaft while hoisting each time the kelly bar and bucket are raised from the excavation.
It is another objective of this invention to minimize the needed turning force required to rotate the over-reamer.
These and other objectives of the invention will be incurred to those skilled in the art in view of the preferred embodiment and the claims.